# Magnesium dosage: how much to take and when Source: https://magnesiumguide.uk/how-to-take/dosage-and-timing/ Author: Nell Hartley ยท Updated 2026-11-04 **Short answer:** For most adults, 100 to 300 mg of elemental magnesium a day is a sensible supplement dose, and the NHS says up to 400 mg a day is unlikely to cause harm. Trials for sleep used 250 to 500 mg, for blood pressure 300 to 450 mg and for migraine about 600 mg. Doses above 400 mg are best agreed with a GP or pharmacist. ## How much magnesium do trials use? These are the doses of **elemental** magnesium used in the trials behind each claim, not recommendations for everyone. | Purpose | Dose in trials | Length | Evidence | | --- | --- | --- | --- | | [Sleep](/benefits/sleep/) | 250 to 500 mg a day, evening | 3 to 8 weeks | Moderate | | [Blood pressure](/benefits/blood-pressure/) | 300 to 450 mg a day (median 368 mg) | 1 to 6 months | Moderate | | [Migraine prevention](/benefits/migraine/) | about 600 mg a day, split | 12 weeks | Moderate | | [Anxiety and stress](/benefits/anxiety-and-stress/) | 250 to 300 mg a day | 6 to 8 weeks | Weak | | [Constipation](/benefits/constipation/) | 1.5 g magnesium oxide a day (about 900 mg magnesium) | 4 weeks | Strong | ## What is the safe upper limit? The NHS says 400 mg a day or less of magnesium from supplements is unlikely to cause any harm, and that higher doses for a short time can cause diarrhoea. The European Food Safety Authority set a stricter upper level of 250 mg a day for supplements, chosen to avoid mild diarrhoea in a small share of people. Magnesium from food doesn't count towards either limit. Doses above 400 mg, such as those used for migraine, are best discussed with a GP or pharmacist first. If you have kidney disease, don't take magnesium supplements unless your doctor recommends them. ## How do you work out the dose from the label? 1. Find the **elemental magnesium** per serving, usually in the nutrition table ("Magnesium: 200 mg"). 2. Check how many capsules make a serving. 3. If the label gives only the compound weight, multiply by the form's magnesium share: about 14% for glycinate, 16% for citrate, 60% for oxide. Our [buying guide calculator](/buying-guide/) does this for you. ## When is the best time to take magnesium? - **For sleep:** in the evening, as in the trials, with or after your evening meal. - **For everything else:** whenever you will remember. Consistency matters more than timing. - **With food:** usually better, to reduce stomach upset. - **Away from some medicines:** leave two to four hours between magnesium and some antibiotics, osteoporosis tablets and levothyroxine. See [medicines it affects](/safety/drug-interactions/). ## How should you start taking magnesium? Start low, at 100 to 200 mg a day, and increase only if needed. If you get loose stools, lower the dose, split it, or switch to [magnesium glycinate](/forms/magnesium-glycinate/). Reassess after a month. ## How long does magnesium take to work? It depends on the reason. As a laxative, within hours. For sleep, the trials measured results after three to eight weeks. For blood pressure, effects showed after about a month, and for migraine prevention the trials judged results after 12 weeks. If you're taking it because a blood test showed low magnesium, your GP will usually recheck after a few weeks. If you've noticed no difference after the trial length for your purpose, it's reasonable to stop. [Foods high in magnesium](/basics/food-sources/) are a good long-term alternative. ## Frequently asked questions ### How much magnesium should a woman take a day? The UK reference intake for women aged 19 and over is 270 mg a day from all sources, mostly food. If you take a supplement to top up, 100 to 200 mg is usually enough. ### Can I take 500 mg of magnesium a day? Some trials used doses around 500 mg, but it is above the 400 mg the NHS says supplements are unlikely to cause harm at, and diarrhoea becomes more likely. Check the label: 500 mg of a magnesium compound may contain far less magnesium. ### Is it better to take magnesium once or twice a day? For doses above about 200 mg, two smaller doses with meals are easier on the gut and may be absorbed a little better. ### What happens if I take too much magnesium? Usually diarrhoea, stomach cramps and nausea. Very high doses, especially in people with kidney disease, can cause dangerously high blood magnesium, with weakness, low blood pressure and an irregular heartbeat. Seek urgent advice if that happens. ### Can I take magnesium every day long term? For most healthy adults, yes, at up to 400 mg a day from supplements. Long-term high-dose laxative use, kidney disease and older age need more care, so ask your GP if any apply. ## References 1. [NHS. Vitamins and minerals: others (magnesium).](https://www.nhs.uk/conditions/vitamins-and-minerals/others/) 2. [Public Health England. Government dietary recommendations, 2016.](https://assets.publishing.service.gov.uk/media/5a749fece5274a44083b82d8/government_dietary_recommendations.pdf) 3. [EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for magnesium. EFSA Journal, 2015.](https://www.efsa.europa.eu/en/efsajournal/pub/4186) 4. [Schuster J et al. Magnesium bisglycinate supplementation in healthy adults reporting poor sleep: a randomized, placebo-controlled trial. Nature and Science of Sleep, 2025.](https://pubmed.ncbi.nlm.nih.gov/40918053/) 5. [Abbasi B et al. The effect of magnesium supplementation on primary insomnia in elderly: a double-blind placebo-controlled clinical trial. Journal of Research in Medical Sciences, 2012.](https://pubmed.ncbi.nlm.nih.gov/23853635/) 6. [Zhang X et al. Effects of magnesium supplementation on blood pressure: a meta-analysis of randomized double-blind placebo-controlled trials. Hypertension, 2016.](https://pubmed.ncbi.nlm.nih.gov/27402922/) 7. [Peikert A et al. Prophylaxis of migraine with oral magnesium: results from a prospective, multi-center, placebo-controlled and double-blind randomized study. Cephalalgia, 1996.](https://pubmed.ncbi.nlm.nih.gov/8792038/)